Allow users to only edit some pages and their children

This code checks for the parent and allows a logged-in user to edit this pages and all descendants of that page. Just edit two parts: The user ID and the parent Page ID and you are good to go.

Via

function check_allow_edit ( $caps, $cap, $user_id, $args ) {

	$to_filter = [ 'edit_post', 'delete_post', 'edit_page', 'delete_page' ];

	// If the capability being filtered isn't of our interest, just return current value
	if ( ! in_array( $cap, $to_filter, true ) ) {
		return $caps;
	}
	
	// First item in $args array should be page ID
	$page_id = $args[0];
	$page = get_post( $page_id );

	/*
	if ( ! $current_page_id ) {
		//cannot edit
		$can_edit = false;
	}
	*/
	if ($page->post_parent)	{
		$ancestors = get_post_ancestors($page->ID);
		$root = count($ancestors)-1;
		$page_id = $ancestors[$root];
	}

	if ($user_id == 137) { //change to the user ID you want

		$allowed_ids = array('37'); //allow this page and children of it

		if (in_array($page_id, $allowed_ids)) {
			//user 1 and current page is allowed - can edit
			$can_edit = true;
		} else {
			//user 1 and current page is not allowed - cannot edit
			$can_edit = false;
		}

	} else {
		//not the specified user so edit all
		$can_edit = true;
	}

	// First item in $args array should be page ID
	//if ( ! $args || empty( $args[0] ) || ! wpse_user_can_edit( $user_id, $args[0] ) ) {
	if ( ! $args || empty( $args[0] ) || $can_edit == false ) {	
		// User is not allowed, let's tell that to WP
		return [ 'do_not_allow' ];
	}
	// Otherwise just return current value
	return $caps;

};

add_filter( 'map_meta_cap', 'check_allow_edit', 10, 4 );